This invention relates to write equalization techniques in magnetic tape recording.
Magnetic tape drives store digital information as transitions of magnetic flux on the surface of a magnetic tape. These transitions are separated by varying lengths, the ratio between the maximum and minimum lengths varying according to the data storage standard being used. For example, the 1,7 run length limited standard, 1,7 RLL, calls for a minimum separation of one binary "0" between transitions (binary "1's") and a maximum separation of seven "0's" between transitions ("1's"). Thus, the number of clock cells between transitions using the 1,7 RLL standard varies between a minimum of two and a maximum of eight clock cells.
In reading data from a magnetic tape, a read head passes over the flux transitions. The flux transitions induce a signal in the read head. The amplitude of the signal induced in the read head varies substantially depending on the transition separation, with the signal amplitude of an eight cell separation being much greater than that of a two cell separation. It is desirable to lessen differences in the amplitude of the signals induced in the read head by the minimum and maximum separations. The process of such minimization is known as equalization.
Originally, all equalization was accomplished on the readback of the data. Accomplishing the entire equalization process during the readback made the readback process and hardware more complicated, and introduced a greater likelihood of errors during the readback operation. For that reason, write equalization has been developed to split the burden of equalization of the data separations between write and readback operations.
A magnetic recording device writes data on a magnetic tape by passing a current through the write head. The write head creates a magnetic field which induces a magnetic alignment on the surface of the recording medium.
The data pattern that is written on the magnetic tape can be understood by considering the write current waveform passed through the write head. When, for example, the 1,7 RLL data recording standard is used, the direction of the write current flowing through the write head changes every time a binary "1" is to be written on the tape. For example, a binary "1" bracketed by binary "0's" will be written by a change in the direction of the write current in the write head at the binary "1" preceded and followed by periods in which the write current flows in opposite directions. Data is read back from a magnetic tape by signals induced from magnetized areas of the tape passing beneath the read head. The amplitude of the signal induced in the read head varies with the length of a region in which no transition occurs, such as a string of consecutive "0's". Long strings of binary "0's" will thus induce a readback signal having a progressively larger amplitude. Short strings of binary "0's" will similarly induce a readback signal having smaller amplitude. This variation in readback signal amplitude is undesirable.
Write equalization has been previously achieved by inserting write equalization pulses, each consisting of a pair of alternating transitions in the write current between changes in current direction that represent data. A write equalization pulse is a short-duration change in the direction of the write current, and is used to "cut a slice" out of the area bounded by the write current waveform.
While write equalization has been beneficial, it has not been fully effective in equalizing the amplitude of readback signals. Moreover, due to frequency response limitations in the write electronics and the write head, the amount of equalization achieved by a fixed write equalization pulse will vary. It is desirable, therefore, to vary the characteristics of the write equalization pulses to be able to compensate for changes in system requirements. One previously proposed approach for improving write equalization involves varying the pulse width of the write equalization pulses. That approach, however, can create phase distortions in the readback signal.